Official Luthiers Forum!

Hi- Over the past year I've been collecting wood for my first acoustic build (000 size) and building some jigs that I figure I'll need once I start construction. Looking at the back plates recently, I noticed one of them is warped (maybe cupped is technically the correct term). It's cupped about 1/8" in the center of it's 8 1/2" span. The other plate might be about 1/32", so not too concerned about that one. They're Honduran mahogany and about .200" thick right now.

So, I'm wondering, once I thickness the plates down to .100-.125" or so, will they limber up and the cupping won't be much of an issue? Or is this a big problem? The humidity here in OK is a little low right now (~40%), maybe it will flatten out once we get into spring/summer?

Comments? Suggestions?

Thanks!

Not a problem.

It should be fine. When you get it to building thickness and then brace it you shouldn’t find it fighting against you. But pay careful attention to humidity control at that stage if the build. Bad things happen if the wood is too humid (or dry) when you glue up the bracing, or close the box.

I'm 100% with what Mark and Ed said. Especially the part about taking care not to brace or close when humidity is out of whack (technical term), although you can usually get away with being on the dry side, but not the wet.

_________________
Joe Beaver
Maker of Sawdust

For me - the "worst" place for warp is on the glue line - because it makes aligning the glue line more difficult. But a little warp that flattens quickly under light pressure is no problem.

If you are concerned - iron the offending piece flat with a hot clothes iron and a little water. Once it "flattens" (you will feel it relax) - pile some heavy junk on top of it to keep it flat for a while. Works like a charm.

If there is a mistake to be made, I've probably made it.

Somewhere along the way I discovered the true test wasn't how well you built a guitar, but how well you were able to recover from the many follies.

_________________
Joe Beaver
Maker of Sawdust

Piling on here:

1. That type of warping can be managed. Not a huge problem.

2. The higher humidity that you are hoping for is actually something to worry about. The worst of all worlds is when you brace the plates and close up the box in high humidity. When the winter comes, that moisture leaves the wood and the chances of cracks in the plates go way up. WAY up. I was not smart enough about relative humidity for my first several builds, and I have the top cracks to remind me of that folly. Before you move forward with any bracing, get on top of accurately measuring (harder than you think) and controlling relative humidity.

You may find too that when thinning the plates the side that you plane or sand on will cup. Flipping it over will reverse that cupping direction and even it out. In the end though you are just going to arch the plates anyway so you may as well brace the side that is 'pre-cupped' so to speak. 40%RH imho is perfect conditions for bracing.

Let me guess....the concave side was exposed more than the other side. With low humidity, the exposed side shrinks more, causing the cupping. If that is the case, all you have to do is turn it over.

_________________
John